The use of chlorine dioxide in the treatment of waste water for disinfection is well known and is known to have advantages over the use of chlorine. Relatively small quantities of chlorine dioxide are required in such use, typically 500 to 2000 lbs/day in a municipal sewage treatment plant of a medium-sized community.
In U.S. Pat. No. 4,250,159, assigned to the assignee herein, there is described an automated small scale chlorine dioxide producing plant wherein sodium chlorate solution is reacted with sulphur dioxide at subatmospheric pressure by countercurrent flow in a reaction tower, to form an aqueous chlorine dioxide solution having a chlorine dioxide concentration of about 1 to 3 gpl.
This prior art procedure suffers from several drawbacks. The prior art process has a relatively low yield of chlorine dioxide of about 60 to 70% based on chlorate, compared with other chlorine dioxide generating processes, which have the potential for much higher yields.
This prior art process also effects the production of chlorine dioxide in the presence of a small amount of air, which necessitates the use of an absorption tower to achieve dissolution of the chlorine dioxide. Many process control elements are required to automate the prior art process for automatic maintenance-free operation. These factors lead to a relatively high capital cost for the plant relative to the chlorine dioxide output.
Other chlorine dioxide producing reactions exist which potentially may be used to generate chlorine dioxide for waste water treatment in rapid, effective and efficient manner. Canadian Pat. No. 543,589, issued to the assignee herein, describes a chlorine dioxide-producing process wherein sodium chlorate, sodium chloride and sulphuric acid at high acidity, in range of 7 to 13 normal, are reacted together at about 30.degree. C. at atmospheric pressure to form chlorine dioxide and chlorine. Air is passed through the reaction medium to strip the gaseous products and to form an air-diluted product gas stream.
Although the latter procedure generally produces chlorine dioxide at a higher yield than the process of U.S. Pat. No. 4,250,159, the process does, however, suffer from certain drawbacks which render it unsuitable for use in an automated water treatment operation. The large volume of diluent air which is present in the product gas stream necessitates a correspondingly large absorption tower to enable the chlorine dioxide to be dissolved in water. Further, the process requires a relatively long residence time of reaction medium in the generator to achieve high yields of chlorine dioxide, resulting in a large volume of generator vessel. Both of these factors result in a relatively high capital cost for the plant relative to the chlorine dioxide output.
Although air is used as diluent gas for the chlorine dioxide in this prior procedure, the process is known to be prone to "puffing", that is the spontaneous decomposition of chlorine dioxide. This feature of the process is undesirable in an unattended automated operation, in view of the potential for equipment damage and the release of noxious gases.
Canadian Pat. No. 825,084, issued to the assignee herein, describes a similar process to that of Canadian Pat. No. 543,589, except that the high acidity reaction medium is maintained as its boiling point under a subatmospheric pressure to result in a product gas stream which contains steam as the diluent gas for the chlorine dioxide and chlorine and the deposition of a sodium acid sulphate in the reaction vessel.
Sodium acid sulphates are difficult to handle physically and exhibit deliquescence. The formation of this salt in a small scale plant which is intended to be relatively maintenance free is a significant drawback. Further, the deposition of a salt in the reaction vessel necessitates the removal of the same in slurry form, the use of a filter to separate crystals from reaction medium, and a recycle pump and recycle pipes for the separated reaction medium. All these items are a significant factor in the overall capital cost of a plant embodying the process of Canadian Pat. No. 825,084.